obj.instance_eval and mod.module_eval(mod.class_eval) usage

class C
  def initialize
    @x = 1
  end
end

c = C.new
c.instance_eval { puts @x }
puts c.instance_variable_get "@x"

C.class_eval do
  def some_method
    puts "Created in class_eval"
  end
end
c = C.new
c.some_method

Ruby instance_eval example

class Klass 
  def initialize 
    @secret = 99 
  end 
end 
k = Klass.new 
p k
p k.instance_eval('self')
k.instance_eval(%q)
p k.instance_eval { self }
puts k.instance_eval { @secret }
puts k.name
p k.methods(false)

k.instance_eval do
  def first_name
    'yu'
  end
end

puts k.first_name

# Evaluates a string containing Ruby source code, or the given block, within the con- 
# text of the receiver (obj). To set the context, the variable self is set to obj while the 
# code is executing, giving the code access to obj’s instance variables.

k.instance_variable_set(:@secret, 100)
puts k.instance_variable_get(:@secret)

p k.instance_variables

# >> #
# >> #
# >> #
# >> 99
# >> test
# >> ["name"]
# >> yu
# >> 100
# >> ["@secret"]